Process for chilling stream of gas-suspended particles

ABSTRACT

The process described herein comprises the mixing of a chilled gas into a stream of gas-suspended particles. Preferably the chilled gas is the same as the gas in which the particles are suspended and the particles are electrostatically charged. A novel device is described which provides a chamber extending lengthwise of the device into which channels feed the chilled gas into the particle suspension. The chilled gas is introduced from an external cooling system with the gas being advantageously cooled during passage through a coil whose outer walls are cooled by liquid nitrogen or dry ice. This process and device are of particular utility in a system for the spraying of electrostatically charged particles.

This is a division of pending application Ser. No. 821,801, filed Jan.23, 1986, now U.S. Pat. No. 4,691,865.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process and apparatus for the chilling of astream of gas-suspended particles. More specifically it relates to theintroduction of a supercooled gas into a stream of gas-suspendedparticles. Still more specifically it relates to a process and a deviceparticularly suitable for chilling a gas suspension of electrostaticallycharged particles.

2. Description of the Prior Art

U.S. Pat. No. 3,279,936, issued to Clarence W. Forestek on Oct. 18,1966, is directed to the treating of metal surfaces having crevices orpores therein with perflourocarbon polymer particles so as to depositsuch particles into the pores. By having the metal at a raisedtemperature to enlarge the pores and the particles chilled to contactthem, the particles are interlocked in the pores when the particles andthe metal come to an equilibrium temperature whereby the particles areexpanded and the pores contracted.

U.S. Pat. No. 4,051,275, issued on Sept. 27, 1977 was also granted toClarence W. Forestek and is directed to applying such chilled particlesin a fluid stream carrying finely divided particles applied at apressure of 30-120 psi to compact previously deposited and thereafterdepositing additional particles in the space freed by the compaction,said additional particles being a temperature at least 100° F. below thetemperature of the metal surface so that these additional particles willalso be locked into the fissures upon reaching equilibrium temperature.

OBJECTIVES OF THE INVENTION

It is an objective of this invention to aid in the deposition of suchchilled finely divided particles into crevices and pores in the surfaceof metal objects, particularly large objects, by placing anelectrostatic charge on these particles.

It is also an objective of this invention to produce a chilling effecton the electrostatically charged particles prior to application of thesame on the said metal surface.

It is also an objective of this invention to effect a chilling effect ona gas suspension of electrostatically charged particles by the injectionof a supercooled gas in the said suspension prior to application of theparticles onto the said metal surface.

It is also an objective of this invention to design an effective devicefor feeding supercooled gas into the said gas suspension ofelectrostatically charged particles.

SUMMARY OF THE INVENTION

In accordance with the present invention it has been found that finelydivided particles of perfluorocarbon polymer or graphite, molybdenumsulfate, boron nitride or other suitable particles may beelectrostatically charged and then chilled to an appropriately lowtemperature by the introduction of a supercooled gas into the suspensionof particles. Preferably the supercooled gas is the same type of gas asthat used as the suspension medium but this is not critical and othersuitable gases may be used. A device for effecting the introduction ofthe supercooled gas in the particle suspension is also a part of thisinvention and is described in detail hereinafter.

A companion application filed simultaneously herewith describes andclaims the process and apparatus for spraying chilled electrostaticallycharged particles onto rotating large metal objects to effect thedeposition and compacting of particles in heat enlarged surface pores ofthe said object and thereby cause interlocking of the particles in thepores as described in the above-cited U.S. Patents.

The electrostatic charge applied to the particles is advantageously inthe range of 50-150 kilovolts, preferably about 100 kv. Preferably, theparticles are given a positive charge and the object is given a minuspotential. However, while this system is preferred, the reverse systemof charging a particularly advantageous device is that described herein.This device comprises a spray gun, such as used for electrostaticallyspraying paint which has an interior channel through which is passed thespraying medium, in this case the suspension of particles disposed indry air or inert gas, such as nitrogen, carbon dioxide, etc. Thissuspension of particles is passed between electrodes which generate anappropriate electrical field to produce the desired electrostaticcharges on the particles. Appropriate electrical charge for theelectrodes is maintained by a high voltage transformer and condensers.

The electrostatically charged particle suspension is passed into thechilling apparatus in which a supercooled gas is introduced into thesuspension. A cooling medium such as liquid nitrogen or solid carbondioxide is maintained is an adjacent vessel so that supercooling can beeffected on the gas which is to be introduced into the gaseoussuspension medium. The supercooled gas, preferably the same as used inthe original suspension, is introduced into the suspension ofelectrostatically charged particles during its passage through a chamberor passageway in the chilling apparatus. The supercooled gas introducedinto the suspension is desirably maintained at -20° C. (-4° F.) to -130°C. (-202° F.), preferably -50° C. (-58° F.) to -120° C. (-184° F.).

The spray gun used for applying the electrostatic charge to theparticles may be any one of a number of commercial spray guns suitablefor spraying electrostatically charged particles. A particularlysuitable spray gun is the one marketed under the trademark "GEMA" whichhas a four electrode and high voltage generating system including a highvoltage transformer and a high voltage set of condensers. The coolingattachment is fitted onto the front or spraying end of the spray gun sothat as the spray of suspended electrostatically charged particlesemanates from the spray gun it passes through a mixing chamber in thechilling apparatus and then out over impact spreading cone.

The polymer suspension stream is advantageously ejected from the spraygun at a pressure of 15-150 psi (1-10 atmosphere). A suspension ofparticles in the suspension medium is advantageously kept in a storagetank in which the particles are stirred up by feeding in nitrogen orother gas. When the stream is to be applied, the appropriate valve isopened to allow the gas to escape with particles suspended therein andcarried to the electrostatically charging chamber of the spray gun priorto mixing with the chilling gas medium. Various other means may be usedto feed polymer particles into the gas stream such as Venturi, screwfeed, blower, pump, etc.

For particles of 4.7⁻¹⁰ gm or 5 microns in size and a voltage of about80-100 kilovolts applied between the gun and the object it is estimatedthat the particles have an initial velocity of about 11 meters persecond and about 0.09-0.10 meters per second in the vicinity of theworkpiece. Smaller sized particles, e.g., of less than 1 micron are morefavorable since the ultimate speed due to the electrical feed will behigher than for larger particles.

In applying the electrostatic charge it may be advantageous to have aseries of electrodes spaced in pairs along the path of the suspendedparticles with a voltage of approximately 20 kilovolts applied betweeneach pair of electrodes and applied transversely to the direction offlow of the gas.

SPECIFIC EMBODIMENT OF THE INVENTION

The description of the process and apparatus of this invention arefacilitated by reference to the drawings.

FIG. 1 is an elevational front view of a preferred modification of thedevice and system for effecting cooling of the particles suspension.

FIG. 2 is a side cross-sectional view of the device of FIG. 1 taken atline 2--2.

FIG. 3 is a side elevational view of a spray gun adapted by the deviceof FIGS. 1 and 2 to effect cooling of a gas stream with suspendedparticles and to supply electrostatic charge to the particles.

FIG. 4 is a top view of the front portion of a spray gun shown in FIG.3.

FIG. 5 is a side cross-sectional view of the front portion of the spraygun shown in FIGS. 3 and 4 taken at line 5--5 of FIG. 4.

FIGS. 1 and 2 show the spray gun attachment 19 which is designed toeffect cooling of a gas stream carrying finely divided particles whichare already electrostatically charged or about to be electrostaticallycharged. The cooling nozzle 15 is preferably made ofpolytetrafluoroethylene (PTFE). Metal ring or band 16 slides over theexterior of a portion of the spray gun attachment 19 to hold gasentrance duct 20 in position. Cold gas such as nitrogen is fed into thespray gun from cooling tank 21 which preferably consists of a rigid,expanded polyurethane foam in which a heat exchange coil 22 ispositioned in cooling chamber 23. The coil is advantageously made ofcopper tubing having a diameter of about 10 mm. Cover 24 closes thechamber to reduce the admission of heat. The coolant contained in thecooling chamber is advantageously either dry ice (solid carbon dioxide)or liquid nitrogen. When the gas inlet line 25 is opened, gaseousnitrogen, or other preferred gas, flows through cooling coil 22 and thecooled gas (preferably at -70°--140° C.) flows through hose connector 26to gas entrance duct 20 attached to and communicating with the recessedportion 20' running around the circumference of the mixing chamber 15and feeding cold gas through channels 27 into the interior of chamber 15of cooling attachment 19.

FIGS. 3, 4 and 5 show the equipment of FIGS. 1 and 2 attached toelectrostatic spray gun 18. High voltage preferably in the range of70-120 kv is developed in generator 32 and transmitted through lines 33into the interior of the spray gun through switch 34 to high voltagecondenser cascade 35 and then through high voltage transformer 36 toelectrodes 37. Meanwhile particles of the desired size and type arestored in reservoir 38 maintained in suspended state by gas admitted byline 39 at the bottom of the reservoir from which the gas stream conveysthe suspended particles through hose 40 into the spray gun where theyare electrostatically charged as they pass between electrodes 37 intomixing channel 15 where the particles are chilled to the desired lowtemperature by mixing with the cold gas being admitted through duct 20.

The mixing device of this invention is found to operate very efficientlyin chilling the electrostatically charged particles emanating from anelectrostatic spray gun. It is preferable to have the cooling equipmentpositioned after the spray gun. Otherwise the chilled gas suspensionwould have to pass through the electrostatic charging equipment therebychilling the electrodes and other parts coming into contact with thechilled gas suspension and also effecting some rise in temperature inthe chilled suspension. However where such effects are not critical ormay be offset the chilling equipment may be positioned prior to theelectrostatically charging apparatus. In such cases obvious appropriatechanges would be made in the positioning and arrangement of theapparatus described herein. Nevertheless the cooling device will havethe same essential elements described herein.

While certain features of this invention have been described in detailwith respect to various embodiments thereof, it will of course beapparent that other modifications can be made within the spirit andscope of this invention, and it is not intended to limit the inventionto the exact details shown above except insofar as they are defined inthe following claims.

The invention claimed is:
 1. A process for the chilling of a first gasstream having therein suspended finely divided solid particlescomprising the step of introducing into said first stream a secondstream chilled to a temperature in the range of -20° C. to -130° C. 2.The process of claim 1 in which said suspended solid particles areelectrostatically charged.
 3. The process of claim 2 in which saidsuspended solid particles are polymeric perfluorocarbon.
 4. The processof claim 2 in which said suspended solid particles are boron nitride. 5.The process of claim 1 in which said temperature is in the range of -50°C. to -120° C.
 6. The process of claim 2 in which said temperature is inthe range of -50° C. to -120° C.
 7. The process of claim 3 in which saidtemperature is in the range of -50° C. to -120° C.
 8. The process ofclaim 4 in which said temperature is in the range of -50° C. to -120° C.